CHICAGO (Reuters) - Women who put on a lot of weight at any stage of adulthood increase their risk of breast cancer, likely because the hormone estrogen accumulates in the acquired fat and promotes tumors, researchers said on Monday.

Women who became overweight or obese had 1.4 times the risk of breast cancer compared to women whose weight remained stable or declined, their study found.

"The present findings indicate that the relations of adult weight gain to breast cancer is evident throughout the entire adulthood life span rather than being limited to a specific time in life," Jiyoung Ahn of the U.S. National Cancer Institute wrote in the Archives of Internal Medicine.

"These findings may reinforce public health recommendations for the maintenance of a healthy weight throughout adulthood as a means of breast cancer prevention," Ahn wrote.

The nearly 100,000 women in the study reported their weights at age 18, 35, 50 and now. Of them, 2,111 developed breast cancer.

On average, women in the study gained more than 34 pounds (15.6 kg) during their adult lives, roughly evenly divided in the ages between 18 and 35, 35 and 50, and 50 and their current age, while 8 percent maintained their weight.

"Women who gained weight or were overweight or obese were more likely to develop advanced disease or hormone receptor-positive tumors," Ahn wrote.

The relationship between weight gain and breast cancer is complicated, researchers say, because the timing of estrogen exposure and levels of the hormone can be hard to pinpoint.

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In this study, for instance, weight gain was less of a risk factor among women who began menstruating relatively early in life or who took hormone-replacement therapy during or after menopause -- both of which acclimated their bodies to more estrogen.

Women in the study who lost weight during their adult lives did not have a lower risk of breast cancer, unlike indications of such an association reported in some earlier studies.

But the study did conclude that weight gain at any stage of adulthood increased breast cancer risk. Some earlier studies have suggested the riskiest time to put on weight was after menopause, when a woman's ovaries stop producing estrogen and fat cells become the primary source of the hormone.

It was unclear from the study whether modest weight gains increased the risk of breast cancer.

In the United States, there will be an estimated 178,000 new cases of breast cancer this year, with 40,000 deaths.